Automated systems are used for mixing all types of materials, such as concrete mixes contained in ready-mix delivery trucks. Such automated systems measure the energy required for mixing a concrete load contained in a rotatable mixing drum thereby to ensure that “slump” values of the concrete (the term “slump” refers to the workability of a concrete mixture) during transport or at delivery are within a desired range. Such automated systems increase the reliability and consistency of the concrete during transportation and delivery by controlling the duration and rate of rotation of the mixing drum.
Once proper mixing of the concrete components is completed, it is important to maintain a minimum mixing drum speed to prevent segregation of components. This ensures even consistency as well as satisfactory strength in the concrete. On the other hand, it is important to avoid excessive drum speeds that would make the concrete overly stiff due to acceleration or advancement of the setting process, or due to deleterious breaking of air cells leading to release of entrained air from the concrete. Thus, automated slump monitoring systems require accurate measurement of the mixing drum speed.
In U.S. Pat. No. 5,752,768 to Assh, an automated mobile mixer system is described, which relies upon magnetic markers on the rotating mixing drum and magnetic detection sensors to measure the speed and direction of the drum. The drum speed is determined by measuring the time interval between the circumferentially spaced markers as they pass by the electro-magnetic sensors which are mounted on a non-rotating portion of the vehicle. (See e.g., US '768 at col. 9, line 62; col. 11, line 55; and FIGS. 1 and 3).
The use of magnetic markers introduces inaccuracies for concrete monitoring systems. Current systems often employ approximately a dozen magnetic markers mounted circumferentially around the drum axis, as not every concrete mixer truck has bolt heads on the drum surface which can be configured for this purpose. In many cases, magnetic markers need to be attached using an adhesive to the outer drum surface. Further, where a sensor or marker is incorrectly placed or becomes dislodged or misaligned by a tree branch, washing brush, concrete material, or other objects, inaccuracies can be introduced into the sensing and measurement operations of the slump monitoring system.
For example, circumferential misalignment or uneven spacing between magnets, or an imbalance of individual magnet strengths among the markers, can introduce variations in speed readings perceived by the concrete slump monitoring system. For example, if magnets or sensors are weak, or the distance between them increases, it becomes difficult to detect the period peaks in the signal generated by the electro-magnetic sensors, and accuracy is lost. A missing magnet can have an even more severe impact on drum speed monitoring.
In U.S. Pat. No. 8,118,473 and WO 2012/024393 A1 to Compton et al. (both owned by the common assignee hereof), magnetic sensing, as well as wireless accelerometers mounted on the concrete mixing drum, are disclosed for measuring drum speed. In addition to using magnetic sensors, one could also measure “ticks of the speed sensor built into the motor (used for rotating the drum)” or could detect signals generated by “an auxiliary processor coupled to a wireless accelerometer” mounted on the mixing drum. (US '473 at col. 21, line 65).
However, while a wireless accelerometer mounted on the concrete drum might be sufficient for speed measurement when the delivery truck is parked or otherwise stationary, large errors can be introduced when the truck is in motion. Inaccuracies can be introduced, for example, when the truck is accelerating, braking, turning hard, or travelling on non-level roads or irregular terrain.
Concrete mixing drums, as seen on ready-mix delivery trucks on the roads today, are not pure cylinders that rotate in a purely parallel or perpendicular direction with respect to the ground. Rather, such mixing drums have an irregular pear-like shape, with angled inner walls upon which are mounted two or more blades spirally-oriented around the drum rotational axis, which is slanted 10-20 degrees with respect to horizontal ground; and the concrete is pushed (downwards at a slant) towards a more bulbous end when the drum is rotated in one direction; or otherwise discharged (upwards at a slant) towards and through the drum opening located at the other (less bulbous) end when the drum is rotated in the opposite direction.